I'm not sure I entirely understand your argument. "We decide it exists, therefore it exists" is the basis of all science and mathematics. We form axioms based on what we observe, then extrapolate from those axioms to form a coherent logical system. While it may be a leap of logic to assume others have consciousness, it's a common decency to do that.

Onto the second argument, when I mean "what signal is qualia" I'm talking about what is the minimum number of neurons we could kill to completely remove someone's experience of qualia. If we could sever the brain stem, but that would kill an excess of cells. We could kill the sensory cortex, but that would kill more cells than necessary. We could sever the connection between the sensory cortex and the rest of the brain, etc. As you minimize the number of cells, you move up the hierarchy, and eventually reach the prefrontal cortex. But once you reach the prefrontal cortex, the neurons that deliver qualia and the neurons that register it can't really be separated.

Lastly, you said that assuming consciousness is some unique part of the universe is wrong because it cannot be demonstrably proven to exist. I can't really argue against this, since it seems to relate to the difference in our experience of consciousness. To me, consciousness feels palpable, and everything else feels as thin as tissue paper.